Multi-determinant trial functions in the determination of the dissociation energy of the beryllium dimer: Quantum Monte Carlo study

Both variational Monte Carlo (VMC) and fixed-node diffusion Monte Carlo (DMC) are used to estimate the dissociation energy of Be,. The effect of using single- and multi-reference trial functions on the quality of the Monte Carlo estimates is investigated, with independent-particle wave functions ranging from a restricted Hartree-Fock (RHF) calculation up to a complete active space self-consistent field (CASSCF) with four valence electrons in 12 active orbitals. It was determined that the best trial function for DMC had a high cutoff for inclusion and included all 2s and 2p orbitals in a CASSCF(4,8) calculation. The best DMC estimate, D-e = 829(64) cm(-1), compares well with the experimental value of 839(10) cm(-1) but is lower than values from the best basis-set calculations. (C) 2006 Wiley Periodicals, Inc.